There is a wide variety of hand pushed carts in use for various purposes. One type of hand pushed cart in general use is the shopping cart employed in retail and wholesale commercial stores. This type of cart has a frame, when viewed from above, in the shape of a trapezoid. The narrow end of the trapezoid defines the front of the frame and the wide end of the trapezoid defines the rear of the frame. Attached to the top of the frame is a carrier for transporting items. Attached to the bottom of the frame are two pairs of wheels. The front pair are casters that provide a turning capability to the cart. The rear pair of wheels is fixed in direction, so as to only allow roll in the intended direction of travel. The wheels are located on the frame as near the corners as practical.
One advantage of this type of cart is that it has the ability to fit one inside the other for compact storage. This requires the rear of the frame to be free of obstruction to allow one cart to be inserted into another identical to it. On the other hand, a number of disadvantages exist with a shopping cart having a conventional design. One disadvantage is the wide turning radius required to turn a corner or to turn around. Another disadvantage is the lack of maneuverability when trying to negotiate a narrow aisle passage or tight space along a display case, often requiring the operator of the cart to lift the rear wheels and slide the cart into a desired position. Both disadvantages are made worse when the shopping cart comes under load. The force and difficulty required to maneuver the cart increases as the load increases, becoming a concern for the elderly, infirmed or person of slight build. Another disadvantage of current shopping cart design is the tendency for the cart to pivot about its rear wheels and roll down an incline perpendicular to an initial longitudinal axis of the cart.
There have been a number of attempts to overcome the problems associated with maneuvering shopping carts. U.S. Pat. No. 6,331,009B1 to Wilkinson describes a shopping trolley with four casters located at the corners of a frame, with a control wheel assembly located near the load center of the trolley. The control wheel assembly includes a fixed wheel capable of only rotating about its hub and a damping means to maintain the fixed wheel in constant contact with a traveling surface. Also provided is a lifting lug to raise the control wheel from the traveling surface and allow the nesting of carts of identical design.
U.S. Pat. No. 4,826,186 to Hagelin describes a hand pushed cart having an “M” shaped bottom frame, when viewed from above. Attached to the frame are four casters, each located at a corner of the frame. A fifth wheel (or pair of wheels), capable of only rotating about the hub axis, is located at or near the load center of the frame. The casters and the fifth wheel do not reside along the same plane, i.e. the front and rear wheels do not touch a traveling surface at the same time. The fifth wheel does not have a damping or spring bias.
The prior art addresses many of the disadvantages present in conventional shopping carts. However, in doing so the complexity of the design has significantly increased. As design complexity increases, so do the maintenance and the replacement cost for each cart.